I am experiencing the following error: 'Received signal 11, exiting...'.
So I tried to patch the executable by replacing '%s' with two spaces (20 20) as mentionned here: http://www.lgdb.org/game/return_castle_wolfenstein (thanks to krzxor for the link).

* x86_64: I was able to start a multiplayer dedicated server and connect to it on 64bit. I needed to use software emulation because I was testing over SSH ( $ wolfmp +set r_allowSoftwareGL 1 +connect 127.0.0.1 ), so there might be an issue with OpenGL or sound, you'll need to be more specific with what type of errors you are getting. That being said, it is possible the client will never completely work correctly on a modern system and it may only be useful for dedicated servers.